optimizing: Refactor statistics to use OptimizingCompilerStats helper

Remove all copies of 'MaybeRecordStat', replacing them with a single
OptimizingCompilerStats::MaybeRecordStat helper.

Change-Id: I83b96b41439dccece3eee2e159b18c95336ea933
diff --git a/compiler/optimizing/select_generator.cc b/compiler/optimizing/select_generator.cc
index 46d0d0e..cb7ade9 100644
--- a/compiler/optimizing/select_generator.cc
+++ b/compiler/optimizing/select_generator.cc
@@ -140,7 +140,7 @@
       block->MergeWith(merge_block);
     }
 
-    MaybeRecordStat(MethodCompilationStat::kSelectGenerated);
+    MaybeRecordStat(stats_, MethodCompilationStat::kSelectGenerated);
 
     // No need to update dominance information, as we are simplifying
     // a simple diamond shape, where the join block is merged with the